The subject matter disclosed herein relates to power converters and, more specifically, to improved control of and/or power conversion from polyphase alternating current (AC) machines during low speed operation.
In recent years, increased demands for energy and increased concerns about supplies of fossil fuels and their corresponding pollution have led to an increased interest in renewable energy sources. Two of the most common and best developed renewable energy sources are photovoltaic energy and wind energy. Other renewable energy sources may include fuel cells, hydroelectric energy, tidal energy, and biofuel or biomass generators. However, using renewable energy sources to generate electrical energy presents a new set of challenges.
Many renewable energy sources provide a variable supply of energy. The supply may vary, for example, according to the amount of wind, cloud cover, or time of day. Further, different energy sources provide different types of electrical energy. A wind turbine, for example, is better suited to provide Alternating Current (AC) energy while a photovoltaic cell is better suited to provide Direct Current (DC) energy. Due to the variable nature of the energy supplied as well as the varying type of energy generated, power converters are commonly inserted between the renewable energy source and the utility gird or an electrical load, if operating independently of the utility grid.
It is known that power converters have inherent losses which prevent all of the power generated by the renewable energy source from being converted to usable electrical energy. At low levels of power generation, the energy losses may be greater than the power being generated by the renewable energy source. The power converter is typically switched off to avoid an operating condition in which the power generation system is actually using more energy than it is generating.
Thus, in order to maximize the efficiency of the power generation system, it is desirable to capture energy generated at low power generation levels and to provide a converter able to efficiently operate at those low power generation levels.